1. Field of the Invention
The invention relates in general to the manufacture of cement clinker in rotary kilns and, in particular, to the method of using mill scale in the production of cement clinker.
2. State of the Art
As stated in U.S. Pat. No. 5,156,676, the literature is replete with processes by which the calcining and clinkering of cement ingredients can be accomplished.
With the great concern for environmental protection today, processes have been developed to utilize waste products of every kind that might pollute the environment. Thus, in commonly assigned U.S. Pat. Nos. 5,494,515 and 5,421,880, incorporated by reference herein in their entirety, processes are disclosed in which steel slag and blast furnace slag, potential polluters of the environment, can be used in the production of cement clinker, thus avoiding a major disposal problem for the manufacturer thereof because of the large volumes of material involved.
In a like manner, during the processing, milling, and/or forming of steel, a waste product, known in the industry as xe2x80x9cmill scalexe2x80x9d, is created. This mill scale presents a potential environmental disposal problem. In 1997, the estimated volume of mill scale was 3,670,000 tons. This mill scale flakes off the steel as it is being formed and contains chemical elements that are necessary in the art of making cement clinker. The mill scale obtained from carbon and alloy steel casting and rolling mill operations has been shown to contain predominantly various iron oxides, and minor or trace amounts of silicon oxide, aluminum oxide, calcium oxide, magnesium oxide, lead oxide, titanium oxide, cadmium oxide, chromium oxide, copper oxide, nickel oxide, manganese oxide, and zinc oxide.
The mill scale, when it is generated often accumulates a coating of hydrocarbons such as oil, grease, and the like. Tests have shown that the hydrocarbons (oil and grease) are in amounts of about 0% by weight to about 1% or more. Such hydrocarbons, of course, are deleterious to the environment and must be remediated. Thus, a first disadvantage in using mill scale in the production of cement clinkers is the necessity of removing any hydrocarbons that may be present.
Further, in the prior art process, as far as is known, when the mill scale is attempted to be used in making cement clinkers, it is first finely ground with the other feed stock materials. Because of the physical composition of mill scale, i.e. flakes, the inventors herein have found that it is not necessary to grind and reduce the particle size. However, in the prior art, it typically is mixed with the other feedstock materials and ground with them. This pulverized mixture is then placed into blending equipment where the mill scale and other feed stock materials experience additional mixing and blending. A substantially uniform blend of feed stock material is highly desired in the production of quality cement clinker. A 100% uniform blend is extremely difficult to achieve and is rarely accomplished. This is true probably because of physical constraints on the blending equipment and each of the constituent feedstock materials is of slightly different size, shape, and density. During the blending process, air, used as a blending agent, can be blown through the ground material, including the mill scale if it is added. However, the various components, and in particular the mill scale, probably because of its particle size and bulk density of at least 125 #/ft3., which is a greater density than any of the feed stock components presently used, have a tendency to segregate. The mill scale, in particular, does not uniformly blend with the other feed stock materials and thus causes a greater variation in the blend of the kiln feed. This segregation of the materials fed into the kiln causes an undesirable non-uniform creation of the cement clinkers. Therefore, a second disadvantage incurred in the prior art process of using mill scale in the production of cement clinker is the difficulty in obtaining a substantially uniform blend of feed stock material because the mill scale has a natural tendency to segregate itself from the other feed stock materials.
After the attempt to blend the mill scale with the other feedstock materials, the xe2x80x9cblendedxe2x80x9d feedstock materials are preheated with sufficient heat to volatilize the hydrocarbons but not destroy them by burning so they are passed to the atmosphere where they act as pollutants. Some States, such as California, using emission restrictions, effectively restrict the use of mill scale in the making of cement clinker in rotary kiln systems because any hydrocarbons that may be associated with the mill scale are not destroyed. Therefore, the cement clinker manufacturers, to keep operating have to purchase and use iron ore to mix with the other feedstock materials. This is not only costly, but it also discourages recycling of mill scale, a product that, recycled in the prior art manner, is potentially environmentally unacceptable.
It would be advantageous to have a system for using mill scale to make cement clinker wherein the detrimental hydrocarbons are destroyed in the process and wherein cement clinkers of substantially uniform quality can be formed.
The present invention provides a process for using mill scale in the production of quality cement clinker without the creation of polluting by-products from any hydrocarbons associated with the mill scale.
Further, the novel invention enables a more uniform quality cement clinker to be formed by avoiding the steps of grinding and blending the mill scale with the remaining feedstock materials.
The novel process is accomplished, in part, by grinding and blending the feedstock materials without the mill scale. The ground and blended feedstock material may then fed into a preheater structure where the temperature of the feed inlet part of the preheater is in the range of from about 500xc2x0 F. to about 1700xc2x0 C. (260xc2x0 C.-927xc2x0 C.) and is sufficiently high to preheat the feedstock material. Thus, any calciner equipment present in the system is separate from (and not a part of) a rotary kiln and is coupled to the preheater structure to receive the preheated feedstock material and raise the temperature of the preheated feedstock material to a range of from about 1500xc2x0 F. to about 2300xc2x0 F. (815xc2x0 C.-1260xc2x0 C.). The preheated and partially-to-predominantly calcined feedstock material is then fed into the feed end of the rotary kiln. The volatilized gases are burned after they have been volatilized by some heat zone such as the calciner itself or the feed end of the rotary kiln. In one preferred embodiment of the invention, the mill scale is also fed directly and separately into the feed end of the rotary kiln along with the feedstock materials. The temperature at the feed end of the kiln may be in the range of about 1500xc2x0 F. to about 2300xc2x0 F. (815xc2x0 C.-1260xc2x0 C.). This temperature profile of the kiln is sufficiently high to not only volatilize the hydrocarbons, but also to combust and destroy the volatilized hydrocarbons. It takes burning with small amounts of oxygen, and not just heat, to destroy the volatilized hydrocarbons.
In another preferred arrangement, the hydrocarbons (a fuel) are volatilized in the feed end of the rotary kiln which has a temperature profile insufficient to burn the volatilized gases. Therefore, they are coupled back to the calciner where coal (or some other fuel) is burned and here the volatilized hydrocarbons (now a combustible fuel) from the mill scale are destroyed by burning (combustion) in accordance with well-known chemical reactions that form carbon dioxide and water vapor (CO2+H2O). The fuel value of the volatilized hydrocarbons from the mill scale allows the use of a proportionally lower amount of the main calciner fuel. This fuel exchange means that the combusted hydrocarbons not only are converted to harmless products that do not contribute to the pollution of the environment but also they reduce usage requirement for the main fuel. The rate at which the mill scale is substantially uniformly fed into the feed end of the rotary kiln is determined by analyzing the quality of the cement clinker being formed and adjusting the feed rate of the mill scale to obtain the desired quality.
In yet another embodiment of the invention, the mill scale is fed directly and separately into a heat zone, such as a calciner, where it is heated to a temperature in the range of from about 1500xc2x0 F. to about 2300xc2x0 F. (815xc2x0 C.-1260xc2x0 C.). This temperature is sufficiently high to volatilize the hydrocarbons and form a combustible fuel. In addition, the burning (combustion) that takes place (in the calciner or in any other area in the system where combustion takes place) reduces the volatilized hydrocarbons (fuel) as explained above to CO2+H2O, harmless products that, as stated earlier, do not contribute to pollution of the environment and replace equivalent amounts of the other fuel that is being used in the calciner.
In still another embodiment of the invention, a more uniform quality cement clinker is obtained even though the hydrocarbons are only volatilized but not burned in the feed end of the rotary kiln because the temperature profile of the rotary kiln is not sufficient to bum the volatilized hydrocarbons. However, the mill scale is introduced into the feed end of the rotary kiln separately from the feedstock material and in a substantially uniform manner such that cement clinker of a substantially uniform quality can be obtained.
Thus, it is an object of the present invention to utilize mill scale in the production of cement clinker in a rotary kiln system containing preheating and/or calcining equipment.
It is also an object of the invention to utilize mill scale in the process of cement clinker manufacture without any additional production of harmful or unwanted pollutants.
It is a further object of the present invention to utilize mill scale in the production of cement clinker in a rotary kiln in which the preheater/calciner unit is in a structure that is separate from, and not within, the rotary kiln. Although the invention is described as used with a dry process rotary kiln, the system can be used with any cement clinker producing system in which the mill scale can be introduced into a sufficiently hot area with sufficient amounts of oxygen to accomplish the burning and destruction of the volatilized hydrocarbons.
It is still another object of the present invention to volatilize and destroy the unwanted mill scale hydrocarbons so that additional unwanted pollutants are not generated.
It is also an object of the present invention to keep the mill scale separate from the blended feedstock as the feedstock is coupled or fed into combustion areas so that any hydrocarbons associated with the mill scale will not only be volatilized but also destroyed by combustion.
It is yet another object of the present invention to feed the mill scale and the blended feedstock material separately into a kiln system so that a more uniform cement clinker may be produced.
Thus, the invention relates to a process for using mill scale, which may have hydrocarbons associated therewith, in the production of cement clinker using a rotary kiln system having a rotary kiln with a feed end and a heat end, comprising the steps of blending and preheating feedstock material; transferring the blended and preheated feedstock material to an area in the rotary kiln system where combustion is taking place; adding an amount of mill scale to the blended and preheated feedstock material in the combustion area where the hydrocarbons are volatilized and consumed by the combustion; and blending and diffusing the feedstock materials and the mill scale from the combustion area as they move from the feed end of the rotary kiln to the heat source end thereby forming cement clinker at the heat end of the rotary kiln.